Photometers of this type are known from the prior art and are used for analyzing a liquid. With the aid of a photometer, the concentration of substances in the liquid to be analyzed can be determined because the amount of the substance contained in the liquid has an effect on the absorption of light in the liquid.
Typically, the liquid to be analyzed is introduced into a vessel which is transparent to light and is irradiated by a light source from outside. The light emitted by the light source enters the liquid through a first, optically transparent, wall of the vessel and traverses the liquid to be analyzed. After the light emitted has traversed the liquid to be analyzed, the light exits from the vessel on the opposite side thereof, passing through a second, optically transparent, wall of the vessel where the light is detected by a detector that detects part of the intensity emitted. Based on the weakening of the light intensity, an indication can be obtained photometrically of the concentration of absorbing substances in the liquid.
It is also known to give the vessel a thin and elongated configuration so that a light guide is formed which is filled with the liquid to be analyzed. In order to avoid intensity losses in the course of the beam path through the liquid to be analyzed, the light is typically irradiated at a suitable angle so that the irradiated light impinges on the wall of the light guide at an angle of total reflection and is totally reflected. This requires the inner wall of the light guide to have an index of refraction that is smaller than that of the liquid to be analyzed. The angle that is appropriate for total reflection here is dependent on the indices of refraction of the wall of the light guide and of the liquid to be analyzed.
In the solutions known from the prior art, it has been found to be a disadvantage that the irradiation of light through the optically transparent vessel wall reduces the light intensity. This makes it impossible to use light sources having a low light intensity or to detect very small concentrations of the absorbing substances since because of the weakening upon entry through the first axial vessel wall, the light intensity is not sufficient for this.
Thus, there is a desire to provide a photometer which operates with lower losses and allows a precise analysis.